The following relates to the nuclear reactor arts, nuclear power generation arts, nuclear reactor instrumentation arts, nuclear vessel feedthrough arts, and related arts.
In nuclear reactor designs of the integral pressurized water reactor (integral PWR) type, a nuclear reactor core is immersed in primary coolant water at or near the bottom of a pressure vessel. In a typical design, the primary coolant is maintained in a subcooled liquid phase in a cylindrical pressure vessel that is mounted generally upright (that is, with its cylinder axis oriented vertically). A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. Primary coolant flows upward through the reactor core where it is heated and rises through the central riser, discharges from the top of the central riser and reverses direction to flow downward back toward the reactor core through a downcomer annulus defined between the pressure vessel and the central riser. In the integral PWR design, the at least one steam generator is located inside the pressure vessel, typically in the downcomer annulus. Some illustrative integral PWR designs are described in Thome et al., “Integral Helical Coil Pressurized Water Nuclear Reactor”, U.S. Pub. No. 2010/0316181 A1 published Dec. 16, 2010 which is incorporated herein by reference in its entirety. Other light water nuclear reactor designs such as PWR designs with external steam generators, boiling water reactors (BWRs) or so forth, vary the arrangement of the steam generator and other components, but usually locate the radioactive core at or near the bottom of a cylindrical pressure vessel in order to increase the likelihood that the reactor core will remain submerged in coolant in a loss of coolant accident (LOCA).
The nuclear reactor core is built up from multiple fuel assemblies. Each fuel assembly includes a number of fuel rods. Spaced vertically along the length of the fuel assembly are grid assemblies which provide structural support to the fuel rods. At the top and bottom of the fuel assembly are an upper end fitting and a lower end fitting, respectively, providing structural support. The fuel assembly also includes guide tubes interspersed among the fuel rods. The guide tubes are welded to the grid assemblies as well as the upper and lower end fittings to form the structural support for the fuel assembly. Multiple fuel assemblies are welded or otherwise attached to each other to form a core, which is contained in a core former. The entire core is supported in a core basket, which may be suspended from the reactor lower vessel flange. Control rods comprising neutron absorbing material are inserted into and lifted out of the guide tubes of the fuel assembly to control core reactivity. Instruments that monitor core conditions (e.g. reactor power, temperature, pressure, flow, neutron flux, etc.) and their accompanying cabling may also be inserted into some of the guide tubes. Generally, a guide tube contains either a control rod or an instrument but not both, due to space limitations.
The instruments and associated cabling are called incores because they are located in the core. While it is preferred that the incores be located in a guide tube in the center of the fuel assembly, they may also be located at the edge of a fuel assembly due to the arrangement of other core components. Generally, not all fuel assemblies contain an incore, and the fuel assemblies that do contain an incore only contain one. Locating these instruments in the reactor core, or anywhere in the vessel, is a challenge because the reactor vessel contains high temperature and high pressure water and the core produces radiation, an inhospitable environment for electronics.
One approach for instrumentation in nuclear reactors uses thimble tubes contained in conduits, as disclosed in, for example, U.S. Pat. No. 5,120,491 to Brown et al., filed Sep. 17, 1991 and U.S. Pat. No. 4,983,351 to Tower et al., filed May 1, 1989. In this approach, thimble tubes housed in conduits run from a seal table through the vessel and into the core. The thimble tubes enter the reactor vessel at either the vessel head or the bottom of the vessel to provide a straight line to the core. Multiple thimble tubes are terminated at the seal table located outside of the reactor, for example in a dedicated compartment separate from the reactor compartment.
A problem with this approach is that a failure of a conduit or a seal table connection can cause an unisolable primary leak. Although the resulting leak would be small and within the capacity of the coolant charging pumps, it may still necessitate reactor shutdown. Another problem with the thimble and conduit design is that, if routed from the bottom of the vessel, a leak from around the vessel penetration would, in the absence of any action, cause the core to become uncovered.
In the case of control rods, the usual approach is to employ control rod drive mechanisms (CRDMs) with motors that are mounted externally at the top of the reactor vessel. There is currently interest in relocating the CRDM motors inside the pressure vessel, so as to facilitate compact small modular reactor (SMR) designs and to eliminate the CRDM as a potential LOCA source. Electrically driven CRDMs in which the motor is located in the vessel are called internal CRDMs. A disadvantage of internal CRDMs is that they necessitate a large number of electrical penetrations through the pressure vessel for the power, control, and signal (e.g., position indicator sensor) lines required for operating the CRDMs. These lines would add further thimble tube/conduit lines, and would further burden the (typically already heavily loaded) seal table.
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